Conventional optical fibers are made from glass materials and widely used as optical signal-transmitting mediums for instrumentation between instruments or in an instrument, for transmission of data, for medical use, for decoration, for transmission of image, etc. However, they are poor in flexibility when their diameters are not small enough. Further, they are relatively fragile and are apt to be broken by impact. Furthermore, they are heavy, because their specific gravity is comparatively large. In addition, the optical fibers themselves as well as their connectors are expensive. Due to these drawbacks, attempt has been made to replace glass materials with plastic materials. The advantages with plastic materials are numerous. For instance, the resulting optical fibers are light, tough and flexible so that their diameters and numerical apertures can be made large. Further, for instance, their handling is easy and can be readily connected to light emitting and/or accepting elements.
In general, a plastic optical fiber comprises a core made of a plastic material having a larger refractive index and a good optical transmission and a cladding made of a plastic material having a smaller refractive index and a high transparency. In this structure, light is transmitted by reflection at the interface between the core and the cladding. A larger difference in refractive index between the plastic materials of the core and of the cladding provides the optical fiber with better optical transmission.
As the plastic material having good optical transmission, amorphous plastics are preferred, examples of which are polymethyl methacrylate, polystyrene, etc. (cf. Japanese Patent Publication No. 8978/1968).
In the production of the plastic optical fiber, while it is essential to make the refractive index difference between the core and the cladding as large as possible, it is also important to take account of adhesivity on the interface between the core and the cladding, influence of foreign materials on the polymer or physical, mechanical and/or chemical properties of the polymer from which the optical fiber is formed.
From this point of view, the optical fiber comprising the combination of polymethyl methacrylate and polystyrene or a certain kind of fluorine-containing polymethacrylate disclosed in Japanese Patent Publication No. 8978/1968 is noteworthy. However, the optical fiber comprising polystyrene has some drawbacks such that light transmitted through it has an inclination to be yellow, that the optical transmission efficiency is reduced particularly in a short wavelength range, and that the optical fiber tends to be deteriorated by light so that the flexibility of the optical fiber, which is inherently less flexible, is further decreased. In addition, the adhesivity between the core and the cladding is not good. Fluorine-containing polymethacrylate resins disclosed in the above Publication have poorer heat resistance than the core material, so that attenuation of light transmission increases and reliability as the light transmitting medium decreases as the temperature is raised. A raw material for the production of fluorine-containing methacrylate requires high technique in its production and purification and is expensive.
To overcome these drawbacks, it is proposed to use, as a cladding of the optical fiber, fluororubber (e.g. Viton (trade mark) LM, a copolymer comprising 60% by mole of vinylidene fluoride and 40% by mole of hexafluoropropene) (cf. Japanese Patent Publication No.8978/1968) or a copolymer comprising vinylidene fluoride and tetrafluoroethylene in a certain specific ratio (cf. Japanese Patent Publication No. 32660/1978 and Japanese Patent Kokai Publication (unexamined) No. 80758/1979). The former fluororubber comprising vinylidene fluoride and hexafluoropropene is tacky, but poor in adhesivity with methacrylic resin of the core. It is easily thermally decomposed so that it is difficult to melt mold such fluororubber. Further, it does not afford sufficient reflectance at the interface between the core and the cladding. Thus, it is not practically attractive. When the optical fiber is produced from a fluororubber having a high molecular weight by a complex melt spinning method, the fluororubber thermally shrinks and the cladding has surface waviness, which significantly deteriorates the optical transmission performance of the optical fiber.
Although the copolymer comprising vinylidene fluoride and tetrafluoroethylene has a low refractive index and good mechanical strength such as good flexing resistance and abrasion resistance. Since it is crystalline, its crystallinity is increased by heat treatment to decrease its transparency and thus optical transmission performance. Further, the adhesivity at the interface between the core and the cladding is deteriorated and reflection loss is increased. The optical transmission performance of this optical fiber may be improved, for example, by quenching it when producing it by the complex melt spinning method. However, the copolymer is further crystallized in a temperature range from 60.degree. to 80.degree. C. which the optical fiber encounters in use. This results in deterioration of the optical transmission performance. In addition, since thermal stability of vinylidene fluoride is insufficient, the optical fiber comprising the core made of the copolymer containing 60 to 80% by mole of vinylidene fluoride should be produced by the complex melt spinning method in a narrow processing temperature range, which is commercially disadvantageous.